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  <title>p0874R0: Syntax to anonymously refer to the current declaration contexts</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
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   <h1 class="p-name no-ref" id="title">p0874R0<br>Syntax to anonymously refer to the current declaration contexts</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2017-11-20">2017-11-20</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://cor3ntin.github.io/CPPProposals/anonymous_context_access/anonymous_context_access.html">https://cor3ntin.github.io/CPPProposals/anonymous_context_access/anonymous_context_access.html</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:corentin.jabot@gmail.com">Corentin Jabot</a>
     <dt>Audience:
     <dd>EWG, SG7
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>A way to refer to the current class or namespae without naming them.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
     <ol class="toc">
      <li><a href="#anonymous-struct-class"><span class="secno">1.1</span> <span class="content">Anonymous struct / class</span></a>
      <li><a href="#resolving-ambiguity"><span class="secno">1.2</span> <span class="content">Resolving ambiguity</span></a>
      <li><a href="#code-injection"><span class="secno">1.3</span> <span class="content">Code Injection</span></a>
      <li><a href="#macros"><span class="secno">1.4</span> <span class="content">Macros</span></a>
     </ol>
    <li>
     <a href="#syntax"><span class="secno">2</span> <span class="content">Syntax</span></a>
     <ol class="toc">
      <li><a href="#templates"><span class="secno">2.1</span> <span class="content">Templates</span></a>
     </ol>
    <li><a href="#code-injection①"><span class="secno">3</span> <span class="content">Code injection</span></a>
    <li><a href="#alternatives-approaches"><span class="secno">4</span> <span class="content">Alternatives approaches</span></a>
    <li><a href="#implementation"><span class="secno">5</span> <span class="content">Implementation</span></a>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>There are cases where referring to a class/struct from within itself is either cumbersome ( macro
definition ), or not possible ( anonymous entities ).
It also forces the code injection/metaclass proposal to inject a name in a class or namespace code
fragment, in order to refer to the injected declaration context.</p>
   <p>This proposal introduces a syntax to refer to the class, struct, union or namespace in which the
syntax is used, independant of the entity’s name.</p>
   <h3 class="heading settled" data-level="1.1" id="anonymous-struct-class"><span class="secno">1.1. </span><span class="content">Anonymous struct / class</span><a class="self-link" href="#anonymous-struct-class"></a></h3>
   <p>There has been a strong interest for being able to provide constructors and destructors for
anonymous classes and structs.</p>
   <h3 class="heading settled" data-level="1.2" id="resolving-ambiguity"><span class="secno">1.2. </span><span class="content">Resolving ambiguity</span><a class="self-link" href="#resolving-ambiguity"></a></h3>
   <p>Having a way to refer to both the current class and the current namespace offers a simple way
to resolve ambiguities between a name that exist both in a class and its enclosing namespace.</p>
<pre class="highlight"><span class="k">namespace</span> <span class="kt">long</span><span class="o">::</span><span class="n">namespace_declaration</span><span class="o">::</span><span class="n">foo</span> <span class="p">{</span>
    <span class="k">constexpr</span> <span class="k">static</span> <span class="kt">int</span> <span class="n">bar</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
    <span class="k">class</span> <span class="nc">foo</span> <span class="p">{</span>
        <span class="k">constexpr</span> <span class="k">static</span> <span class="kt">int</span> <span class="n">bar</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
        <span class="kt">void</span> <span class="nf">f</span><span class="p">()</span> <span class="p">{</span>
            <span class="k">static_assert</span><span class="p">(</span><span class="k">typename</span><span class="p">(</span><span class="k">namespace</span><span class="p">)</span><span class="o">::</span><span class="n">bar</span> <span class="o">==</span> <span class="mi">0</span><span class="p">);</span>
        <span class="p">}</span>
    <span class="p">};</span>
<span class="p">}</span>
</pre>
   <h3 class="heading settled" data-level="1.3" id="code-injection"><span class="secno">1.3. </span><span class="content">Code Injection</span><a class="self-link" href="#code-injection"></a></h3>
   <p><a data-link-type="biblio" href="#biblio-p0712r0">[p0712r0]</a> proposes to introduce a name in a code fragment to refer to the entity in which it will
be injected.
For example <code class="highlight"><span class="o">-></span> <span class="k">namespace</span> <span class="n">Foo</span> <span class="p">{</span> <span class="p">};</span></code> use the placeholder <code class="highlight"><span class="n">Foo</span></code> to refer to the the class in
which it will be injected. This has a few drawbacks. The developer needs to come with a
placeholder name, which will probably end up being <code class="highlight"><span class="n">C</span></code> or <code class="highlight"><span class="n">N</span></code> most of the time.
It make the intent of the code less clear to a reader. Is <code class="highlight"><span class="n">Foo</span></code> the name of the code-fragment ? are
we created an object <code class="highlight"><span class="n">Foo</span></code> in the current scope ?</p>
   <p>These issues can be solved if we have a consistent way to refer to an anonymous class or a current
namespace.</p>
   <h3 class="heading settled" data-level="1.4" id="macros"><span class="secno">1.4. </span><span class="content">Macros</span><a class="self-link" href="#macros"></a></h3>
   <p>Macros that define or declare class constructors, destructors or otherwise manipulate a class'
name from inside the class are quite frequent. Such macros are often used to avoid duplicating
repetitive code stubs, and have been found in large projects such as LLVM and Chromium.</p>
   <p>As they rely on knowing the class name, they have to been defined as <code class="highlight"><span class="cp">#define EvilMacro(ClassName</span> <span class="cp"></span><span class="cm">/*,...*/</span><span class="p">)</span></code> and used in the same fashion.</p>
   <p>Having a consistent way to refer to a class name without an identifier would simplify the use of
such macros.</p>
   <p>And while defining class members in macros is certainly not something that should be encouraged,
it has its uses in large code base.</p>
   <p>Code injection will certainly make the need for such macros less frequent.  However, it has been
identified that a facility akin to the one this paper proposes is one of the features required for
The Qt framework to benefit from code injection without having to modify millions of lines of code.
That is one of the driving motivation behind this proposal.</p>
   <h2 class="heading settled" data-level="2" id="syntax"><span class="secno">2. </span><span class="content">Syntax</span><a class="self-link" href="#syntax"></a></h2>
   <p><code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="k">namespace</span><span class="p">)</span></code> is valid in a scope identifier and refers to the current namespace, or the
global namespace.</p>
   <p>As a scope identifier, <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> refers to the identifier of the current <code class="highlight"><span class="k">class</span></code>, <code class="highlight"><span class="k">struct</span></code>,
or <code class="highlight"><span class="k">union</span></code>.</p>
   <p><code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> can refer to the declaration of a constructor, and <code class="highlight"><span class="o">~</span><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> can
similarly refer to the declaration of the class destructor.</p>
   <p>Constructors or destructors declared using this syntax or the class of the name belong to the same
overload set (and therefore can not be defined with one syntax and later redefined with the other
syntax).</p>
   <p>When it’s not used as the destructor or constructor name, <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> refers to the type of
the <code class="highlight"><span class="k">class</span></code>, <code class="highlight"><span class="k">struct</span></code>, or <code class="highlight"><span class="k">union</span></code> in which it is used.</p>
   <p>In the rest of this document, a <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="typename-id-expression">typename id expression</dfn> refers to either <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> or <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="k">namespace</span><span class="p">)</span></code>.</p>
   <p>Using <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> outside of a <code class="highlight"><span class="k">class</span></code>, <code class="highlight"><span class="k">struct</span></code> or <code class="highlight"><span class="k">union</span></code> is ill-formed.</p>
   <p>Constructors and destructors using this syntax in unions, whether they are anonymous or not,
remains invalid.</p>
   <p>As a <a data-link-type="dfn" href="#typename-id-expression" id="ref-for-typename-id-expression">typename id expression</a> unambiguously refers to the entity in which it is used, whether that
is
the current namespace, or the current record, it can not be preceded by a nested-name-specifier,
nor can it be used through an object. The following expressions are therefore ill-formed:</p>
   <p><code class="highlight"><span class="o">::</span><span class="k">typename</span><span class="p">(</span><span class="k">namespace</span><span class="p">)</span></code> <code class="highlight"><span class="n">Foo</span><span class="o">::</span><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> - ill-formed because a scope-specifier is specified.</p>
   <p><code class="highlight"><span class="n">foo</span><span class="o">-></span><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span><span class="cm">/*...*/</span></code> : ill-formed, call through an object.</p>
   <h3 class="heading settled" data-level="2.1" id="templates"><span class="secno">2.1. </span><span class="content">Templates</span><a class="self-link" href="#templates"></a></h3>
   <p>In a template class definition, <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> behaves like the <em>injected-class-name</em>. <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> can be followed by a template arguments list, like a type-id.</p>
   <p><code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span><span class="o">&lt;</span><span class="n">Foo</span><span class="o">></span> <span class="cm">/*...*/</span></code></p>
   <h2 class="heading settled" data-level="3" id="code-injection①"><span class="secno">3. </span><span class="content">Code injection</span><a class="self-link" href="#code-injection①"></a></h2>
   <p>While this syntax is not directly related to code injection, code injection may benefit from it.</p>
   <p>In a code fragment <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> and <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="k">namespace</span><span class="p">)</span></code> should refer to the scope the
fragment will be injected in, alleviating the need to inject names in the scope of the code
fragment for that purpose.</p>
   <p><a data-link-type="biblio" href="#biblio-p0712r0">[p0712r0]</a> suggests using <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">reflection</span><span class="o">-</span><span class="n">expression</span><span class="p">)</span></code> to get a type from a reflection.
While I believe there are arguably better approaches ( including using the same syntax for code
injection and "reversing" a reflection), this proposal would not prevent that syntaxe, and it would
be cohesive and logical</p>
<pre class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span>
<span class="k">typename</span><span class="p">(</span><span class="k">namespace</span><span class="p">)</span>
<span class="k">typename</span><span class="p">(</span><span class="n">reflection</span><span class="o">-</span><span class="n">expression</span><span class="p">)</span>
</pre>
   <p>These three expression return, depending on the context, either a type, or a scope specifier.</p>
   <p><a data-link-type="biblio" href="#biblio-p0712r0">[p0712r0]</a> further suggests <code class="highlight"><span class="k">namespace</span><span class="p">(</span><span class="n">reflection</span><span class="o">-</span><span class="n">expression</span><span class="p">)</span></code> as a mean to get a scope specifier
from a reflection on a namespace. The same semantic could be achieve with the
typename(reflection-expression) syntax, depending on the context.</p>
   <h2 class="heading settled" data-level="4" id="alternatives-approaches"><span class="secno">4. </span><span class="content">Alternatives approaches</span><a class="self-link" href="#alternatives-approaches"></a></h2>
   <p>Some GCC versions allow <code class="highlight"><span class="k">decltype</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)</span></code> to be evaluated in the context of a static member
declaration. This can be used to the same effect than the proposed <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code>. However this
non standard behaviour relies on an unnatural semantic and does not solve the problem of addressing
the current namespace. It also requires introducing a static member in the class for the purpose of
querying the class type.</p>
   <p>Different names have been discussed, including <code class="highlight"><span class="k">decltype</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code>, <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="k">typename</span><span class="p">)</span></code> <code class="highlight"><span class="k">namespace</span><span class="p">(</span><span class="k">namespace</span><span class="p">)</span></code> or the introduction of new keywords like <code class="highlight"><span class="n">__classname</span></code>. However these
syntaxes either make no sense semantically (<code class="highlight"><span class="k">decltype</span><span class="p">(</span><span class="n">class</span><span class="p">)</span></code>), are absurdly convoluted or are
__ugly.</p>
   <p><code class="highlight"><span class="p">(</span><span class="n">class</span><span class="p">)</span></code> and <code class="highlight"><span class="p">(</span><span class="k">namespace</span><span class="p">)</span></code> could also be viable, more terse expressions. However that requires
expressions such as <code class="highlight"><span class="k">decltype</span><span class="p">((</span><span class="n">class</span><span class="p">))</span></code> to have 2 pairs of parentheses which may be confusing and
error prone.</p>
   <h2 class="heading settled" data-level="5" id="implementation"><span class="secno">5. </span><span class="content">Implementation</span><a class="self-link" href="#implementation"></a></h2>
   <p>A proof-of-concept (for the <code class="highlight"><span class="k">typename</span><span class="p">(</span><span class="n">class</span><span class="o">|</span><span class="k">namespace</span><span class="p">)</span></code> syntax) was implemented in clang and is
available <a href="https://github.com/cor3ntin/clang/tree/typename_id_expression"> on Github</a>.
No particular difficulty was encountered.</p>
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  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#typename-id-expression">typename id expression</a><span>, in §2</span>
  </ul>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-p0712r0">[P0712R0]
   <dd>Andrew Sutton, Herb Sutter. <a href="https://wg21.link/p0712r0">Implementing language support for compile-time programming</a>. URL: <a href="https://wg21.link/p0712r0">https://wg21.link/p0712r0</a>
  </dl>
  <aside class="dfn-panel" data-for="typename-id-expression">
   <b><a href="#typename-id-expression">#typename-id-expression</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-typename-id-expression">2. Syntax</a>
   </ul>
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